Coordinate measuring machines (CMM's) are high precision machines that are used to measure objects, such as automobile body parts. The CMM tests the object to make sure that it has a correct, desired shape and size within a defined tolerance that can be as little as several ten-thousandths of an inch. Typical CMM's have three motor-driven rails, each of which is movable under computer control along one of three mutually orthogonal axes. A probe is positioned at the end of one of the rails. The probe is quickly moved to a position near the object, is brought to a stop, and then is moved slowly at a near constant velocity until it touches the object, called "making a hit". Thus, the rails, and hence the probe, are accelerated and decelerated before being again brought to a low velocity to make the hit.
One type of CMM that is currently available has a horizontal table on which the object is mounted and a single vertical column mounted at a side of the table. A motor-driven drive assembly moves the column along a side of the table in a direction defined as the x-axis. A carriage is mounted on the vertical column, and moves vertically in a direction perpendicular to the table and defined as the z-axis. The carriage has a motor for moving a horizontal rail over the table along the y-axis. The carriage may be moved along a column, where the column is vertically fixed, or the carriage may be mounted on top of the column while the column itself is raised and lowered by a motor.
When the rails of the CMM are moved, and especially when they are stopped and started, vibrations occur. To ensure accuracy in the measurement, these vibrations need to be sufficiently attenuated before making a measurement. While this attenuation can be accomplished simply by waiting for the vibrations to dampen, such an approach greatly reduces the throughput of measurement.
In U.S. Pat. Nos. 5,042,162 and 4,958,437, which are hereby incorporated by reference, a gantry-type CMM is shown and described. In such a CMM, an inverted U-shaped gantry moves along a horizontal table, driven by a motor that is positioned at one side of the table. Those patents describe in detail torques, and hence sources of vibration, that are experienced by a gantry-type CMM, as well as a type of vibration damper that increases throughput by damping vibrations, thus allowing measurements to be taken more quickly in succession. In the CMM described in those patents, one damper is positioned on the gantry, one damper is positioned on a vertical z-rail that has a probe, and one is positioned under the table. The damper that is on the gantry is positioned as far as possible away from the x-rail drive both in the vertical and horizontal directions. In this CMM, the rails are driven with a motor and a multiple v-belt, which runs smoothly and has little vibration.